An Operating System (OS) is an interface between a computer user and computer hardware. An operating system is a software which performs all the basic tasks like file management, memory management, process management, handling input and output, and controlling peripheral devices such as disk drives and printers.

   Some popular Operating Systems include Linux Operating System, Windows Operating        System

• The purpose of an operating system is to provide an environment in which a user can execute programs conveniently and efficiently.
• An operating system is software that manages computer hardware and software. The hardware must provide appropriate mechanisms to ensure the correct operation of the computer system and to prevent user programs from interfering with the proper operation of the system. 
• The operating system (OS) is a program that runs at all times on a computer. All other programs, including application programs, run on top of the operating system.
• It does assignment of resources like memory, processors and input / output devices to different processes that need the resources. The assignment of resources has to be fair and secure.

                                Conceptual View of Computer System

Need of Operating System:

Operating Syatem  as a platform for Application programs: Operating system provides a platform, on top of which, other programs, called application programs can run. These application programs help the users to perform a specific task easily

       Functionalities of Operating System

• Resource Management: When multiple processes run on the system and need different resources like memory, input/output devices, the OS works as Resource Manager, Its responsibility is to provide hardware to the user. It decreases the load in the system.
• Process Management: It includes various tasks like scheduling and synchronization of processes. Process scheduling is done with the help of CPU scheduling algorithms. Process Synchronization is mainly required because processes need to communicate with each other. 
• Storage Management: The file system mechanism used for the management of the secondary storage like Hard Disk.  All the data is stored in various tracks of Hard disks that are all managed by the storage manager.
• Memory Management: Refers to the management of primary memory, mainly allocation and de-allocation of memory to processes. The operating system has to keep track of how much memory has been used and by which process. It has to decide which process needs memory space and how much.
• Security/Privacy Management: Privacy is also provided by the Operating system using passwords so that unauthorized applications can't access programs or data. For example, Windows uses Kerberos authentication to prevent unauthorized access to data.

Operating System have  evolved over the years. So, their evolution through the years can be mapped using generations of operating systems. There are four generations of operating systems. 

The First Generation ( 1945 - 1955 ): Vacuum Tubes and Plugboards

Digital computers were not constructed until the second world war. Calculating engines with mechanical relays were built at that time. However, the mechanical relays were very slow and were later replaced with vacuum tubes. These machines were enormous but were still very slow.

These early computers were designed, built and maintained by a single group of people. Programming languages were unknown and there were no operating systems so all the programming was done in machine language. All the problems were simple numerical calculations.

By the 1950's punch cards were introduced and this improved the computer system. Instead of using plugboards, programs were written on cards and read into the system.

The Second Generation ( 1955 - 1965 ): Transistors and Batch Systems

Transistors led to the development of the computer systems that could be manufactured and sold to paying customers. These machines were known as mainframes and were locked in air-conditioned computer rooms with staff to operate them.

The batch systemBatch System was introduced to reduce the wasted time in the computer. A tray full of jobs was collected in the input room and read into the magnetic tape. After that, the tape was rewound and mounted on a tape drive. Then the batch operating system was loaded in which read the first job from the tape and ran it. The output was written on the second tape. After the whole batch was done, the input and output tapes were removed and the output tape was printed.

The Third Generation ( 1965 - 1980 ): Integrated Circuits and Multiprogramming

Until the 1960's, there were two types of computer systems i.e., the scientific and the commercial computers. These were combined by IBM in the System/360. This used integrated circuits and provided a major price and performance advantage over the second generation systems.

The third generation operating systems also introduced multiprogramming. This meant that the processor was not idle while a job was completing its I/O operation. Another job was scheduled on the processor so that its time would not be wasted.

The Fourth Generation ( 1980 - Present ): Personal Computers

Personal Computers were easy to create with the development of large-scale integrated circuits. These were chips containing thousands of transistors on a square centimeter of silicon. Because of these, microcomputers were much cheaper than minicomputers and that made it possible for a single individual to own one of them.

The advent of personal computers also led to the growth of networks. This created network operating systems and distributed operating systems. The users were aware of a network while using a network operating system and could log in to remote machines and copy files from one machine to another.